Fiber reinforced composite materials formed of reinforcing fibers such as glass fibers, carbon fibers, and aramid fibers and a matrix resin are excellent in mechanical characteristics such as strength and elastic modulus despite of being lighter in weight than competing metal or the like and thus are used in various fields including aircraft members, spacecraft members, automobile members, ship members, constructional materials, and sporting goods. For applications necessitating high performance in particular, carbon fibers, which are excellent in specific strength and specific modulus, are often used as the reinforcing fibers. Thermosetting resins such as unsaturated polyester resins, vinyl ester resins, epoxy resins, phenol resins, cyanate ester resins, and bismaleimide resins are often used as the matrix resin, and among them, epoxy resins, which are excellent in adhesion with carbon fibers, are often used. For applications necessitating high performance, fiber reinforced composite materials containing continuous fibers are used, and prepregs obtained by combining reinforcing fibers and uncured thermosetting resins are widely used as intermediate bases for producing structures.
Transport applications such as aircraft and automobiles and large structural member applications such as windmills are applications in which the excellence in the specific strength and specific elastic modulus of carbon fiber reinforced composite materials are effectively exhibited. In these applications, measures against damage or deterioration caused by harsh natural environments such as temperature changes, weather, and lightning strikes have been considered important, and various methods have been considered.
When carbon fiber reinforced composite materials are used in commercial aircraft or the like, for example, they are exposed to largely different temperature environments between the duration of flight in the upper air and the duration of stay on the ground. Specifically, the upper air is in an extremely low temperature environment of about −50° C., whereas the temperature during the stay on the ground reaches a temperature of 70° C. or more in some regions. Every time takeoffs and landings are repeated, aircraft are repeatedly exposed to such temperature rises and drops between the extremely low temperature and the high temperature. When carbon fiber reinforced composite materials are used in such an environment, the coefficient of linear expansion of carbon fibers is extremely small, and given this situation, a large difference in the coefficient of linear expansion is present between the carbon fibers and the matrix resin, and expansion and contraction caused by the temperature changes add distortion, that is, thermal distortion to the matrix resin. This thermal distortion may cause minute cracks (microcracks) of about a few tens of to a few hundreds of micrometers in a resin part of the carbon fiber reinforced composite material. When repeatedly exposed to loads by temperature changes (hereinafter, may be referred to as a heat cycle) in a range of from high temperatures to low temperatures, the microcracks are likely to occur (refer to Non Patent Literature 1, for example). When environmental fatigue is further added to the resin part in the state where the microcracks have occurred, the microcracks grow to larger cracks, and the cracks can decrease the mechanical characteristics of the carbon fiber reinforced composite materials.
Resin cured products obtained by curing thermosetting resins, especially epoxy resin compositions are generally brittler than thermoplastic resins, and when used as a matrix resin for the carbon fiber reinforced composite materials, internal thermal distortion is likely to cause microcracks. In order to impart resistance to the thermal distortion to the carbon fiber reinforced composite materials using the thermosetting resins, that is, to suppress the occurrence of microcracks caused by the load of the heat cycle and to suppress the development of cracks having occurred, increasing the elongation and toughness of thermosetting resins is an important issue.
As a method to increase the toughness of matrix resins, especially epoxy resin cured products, a method is known that adds rubber to epoxy resin compositions. As the method that adds rubber, a method has been disclosed that uses reactive carboxy-terminated butadiene-acrylonitrile copolymer rubber (CTBN) or nitrile rubber, for example (refer to Patent Literatures 1 and 2, for example). However, this method has disadvantages in that a process is undergone in which, after the rubber is once dissolved in the epoxy resin composition, phase separation occurs at curing, the epoxy resin cured product changes in morphology due to differences in the type of the epoxy resin composition or curing conditions, and a desired effect of increasing toughness cannot be obtained and that a rubber component being partially dissolved in an epoxy resin phase of the epoxy resin cured product brings about an increase in the viscosity of the epoxy resin compositions, a decrease in the heat resistance (glass transition temperature (Tg)) of the epoxy resin cured products, and a decrease in elastic modulus, leading to a reduced degree of freedom in designing the setting of molding conditions and amounts. Some of the amounts and the molding conditions can reduce the mechanical characteristics of the carbon fiber reinforced composite materials such as tensile strength and compressive strength. In view of these circumstances, it has been required to achieve both an increase in the toughness of the resin and the maintenance and improvement of the mechanical characteristics of the carbon fiber reinforced composite materials.
With regard to these disadvantages, in order to suppress the increase in the viscosity of the epoxy resin compositions and the decrease in Tg of the epoxy resin cured products, a method is disclosed that achieves both mechanical characteristics such as compressive strength and microcrack resistance in carbon fiber reinforced composite materials obtained by molding by resin transfer molding (RTM) using core-shell polymer particles that are substantially insoluble to the epoxy resin (refer to Patent Literatures 3 and 4, for example). However, these techniques are designed for RTM molding and are hence limited in the viscoelasticity of the resin. In addition, no solution is presented for a case containing wires having electric conductivity such as metal.
In the carbon fiber reinforced composite materials, because the carbon fibers are used in combination with the matrix resins, which generally have high insulation properties, their electric conductivity is lower than that of metallic materials, and when a large current occurs by a lightning strike, the large current cannot be instantly diffused. As a result, structural members containing the carbon fiber reinforced composite materials can have severer damage to the structural members when receiving lightning strikes than cases using conventional metallic materials. In the use for aircraft or the like, lightning strikes can cause ignition of fuel or adversely affect internal electronic devices. In order to solve these disadvantages, a prepreg is known that combines reinforcing fibers and a metallic mesh (refer to Patent Literature 5, for example), and a method is known that manufactures a structure with a carbon fiber reinforced composite material containing a prepreg that combines a carbon fiber woven fabric in which metallic wires are woven and a matrix resin (refer to Patent Literature 6, for example).
In the above-described composite materials that combine the metallic wires, the carbon fibers, and the matrix resin for the purpose of imparting lightning strike resistance, the metal differs in a coefficient of linear expansion from both the carbon fibers and the matrix resin, and it is considered that the influence of the thermal distortion cause by the load of the heat cycle is more complicated than a system containing no metallic wire and that the possibility of the occurrence of microcracks further increases. Microcracks from interfaces of the metallic wires can occur caused by faulty adhesion between the metal and the matrix resin and deterioration of adhesion along with corrosion of the metallic wires.
In an area (hereinafter, may be referred to as a resin rich part) in which the matrix resin within the carbon fiber reinforced composite material concentrates, the difference in the coefficient of linear expansion is remarkable, and microcracks are likely to occur by the load of environmental fatigue (refer to Patent Literature 3 and Non Patent Literature 1). A carbon fiber fabric base has overwhelmingly large crimping (fibers being wavy) compared to a case in which carbon fibers are arranged in one direction because of crossing of the warp and the weft. As a result, carbon fiber reinforced composite materials molded by stacking a fabric base of two or more axes as a prepreg have many resin rich parts. In other words, the carbon fiber reinforced composite materials containing a fabric base are more likely to cause microcracks by the heat cycle than ones using a unidirectional base. When containing metallic wires having different fiber diameters or the like, the resin rich part may further increase, and microcracks are considered to be more likely to occur. In other words, an important object is to increase the resistance of the carbon fiber reinforced composite materials containing bases containing such a fabric base and metallic wires to environments.
For the purpose of improving the adhesion and bindability of carbon fibers, various sizing agents for carbon fibers have been disclosed. Examples of the disclosed sizing agents include a compound having a plurality of epoxy groups of an aliphatic type, an epoxy adduct of a polyalkylene glycol, diglycidyl ether of bisphenol A, a polyalkyleneoxide adduct of bisphenol A, and a polyalkyleneoxide adduct of bisphenol A with an epoxy group added. However, no sizing agent formed of one type of epoxy compound imparts sufficient adhesion or bindability to carbon fibers. A method using two or more types of epoxy compounds in combination according to required functions has been disclosed in recent years.
For example, a disclosed sizing agent includes two or more epoxy compounds each having a defined surface energy (see Patent Literatures 7 to 10). Patent Literature 7 discloses a combination of an aliphatic epoxy compound and an aromatic epoxy compound. Patent Literature 7 discloses that a sizing agent present in the outer layer in a large amount has an effect of shielding another sizing agent present in the inner layer in a large amount from air, and this prevents the epoxy group form undergoing ring-opening by water in air. Patent Literature 7 also discloses that the sizing agent preferably contains the aliphatic epoxy compound and the aromatic epoxy compound in a ratio of 10/90 to 40/60, and the aromatic epoxy compound is preferably contained in a larger amount.
Patent Literatures 9 and 10 disclose sizing agents containing two or more types of epoxy compounds having different surface energy. Patent Literatures 9 and 10, which have an object of improving adhesion with a matrix resin, do not specify the combined use of an aromatic epoxy compound and an aliphatic epoxy compound as a combination of two or more types of epoxy compounds and provide no general exemplification of aliphatic epoxy compounds selected from the viewpoint of adhesion.
Another disclosed sizing agent contains a bisphenol A epoxy compound and an aliphatic polyepoxy resin in a mass ratio of 50/50 to 90/10 (see Patent Literature 11). However, the sizing agent disclosed in Patent Literature 11 also contains the bisphenol A epoxy compound as an aromatic epoxy compound in a large amount.
A disclosed sizing agent specifying the combination of an aromatic epoxy compound and an aliphatic epoxy compound is a combination of a multifunctional aliphatic compound on the surfaces of carbon fiber bundles and an epoxy resin, a condensate of an alkylene oxide adduct with an unsaturated dibasic acid, and an alkylene oxide adduct of a phenol on the surface of the multifunctional aliphatic compound (see Patent Literature 12).
A disclosed combination of two or more epoxy compounds is a combination of an aliphatic epoxy compound and a bisphenol A epoxy compound as an aromatic epoxy compound. The aliphatic epoxy compound is a cyclic aliphatic epoxy compound and/or a long chain aliphatic epoxy compound (see Patent Literature 13).
A combination of epoxy compounds having different properties has also been disclosed. A disclosed combination contains two epoxy compounds that are liquid and solid at 25° C. (see Patent Literature 14). Furthermore, a combination of epoxy resins having different molecular weights and a combination of a monofunctional aliphatic epoxy compound and an epoxy resin have been developed (see Patent Literatures 15 and 16).
However, the sizing agents (for example, Patent Literatures 13 to 16) containing two or more components practically fail to achieve both the adhesion between carbon fibers and a matrix resin and the stability of a prepreg during long-term storage. The reason is considered as follows: The following three requirements are needed to be satisfied in order to simultaneously achieve the high adhesion and the suppression of the reduction in mechanical characteristics of a prepreg during long-term storage, but a conventional combination of any epoxy resins has failed to satisfy these requirements. Of the tree requirements, the first is that an epoxy component having high adhesion is present in the inner side (carbon fiber side) of a sizing layer, and the carbon fibers and the epoxy compound in the sizing interact strongly; the second is that the surface layer (matrix resin side) of the sizing layer has a function of suppressing the reaction between a matrix resin and the epoxy compound that is present in the inner layer and that has high adhesion to carbon fibers; and the third is that the surface layer (matrix resin side) of the sizing agent necessitates a chemical composition capable of strongly interacting with a matrix resin in order to improve the adhesion to the matrix resin.
For example, Patent Literature 7 discloses a sizing agent having an inclined structure for increasing the adhesion between carbon fibers and the sizing agent, but Patent Literature 7 and any other literatures (for example, Patent Literatures 8 to 11) have no idea that the sizing layer surface simultaneously suppresses the reaction between an epoxy compound having high adhesion to carbon fibers and a component in a matrix and achieves high adhesion to the matrix resin.
Patent Literature 12 discloses a sizing agent including an inner layer containing a multifunctional aliphatic compound and an outer layer containing an aromatic epoxy resin and an aromatic reaction product each having low reactivity. The sizing agent should prevent a prepreg stored for a long period of time from suffering change with time, but the surface layer of the sizing agent contains no multifunctional aliphatic compound having high adhesion, and this makes it difficult to achieve high adhesion to a matrix resin.